1. Field of Invention
The present invention relates to closed-loop methods and apparatus for controlling acceleration and velocity of a stepper motor and more particularly to such methods and apparatus in which the stepper motor switching angle varies.
2. Description to Related Art
Most stepper motors are operated in an open-loop configuration. Given a known and fixed load driven by the stepper motor, a commutation (step) sequence can be developed by a person having ordinary skill in the art which accelerates the load to a desired velocity without a loss of steps. In other words, each time the motor is commutated the rotor is in a position in which torque generated by the electrical field advances the rotor until it is appropriately positioned for the next commutation, and so forth. In such open-loop configurations, if the load is different than that for which the commutation sequence was developed, steps can be lost to the extent that the rotor may not rotate at all.
Closed-loop configurations use feedback to sense rotor position via a conventional shaft encoder. The rotor position information may be utilized to produce each motor commutation. Most such configurations employ a fixed switching angle commutation. In other words, the motor is commutated each time the rotor advances through a predetermined angle. While the motor does not lose steps with this method, it cannot achieve a precise target velocity due to uncontrollable variance in parameters such as motor supply voltage, friction, etc. Also, if the load is different than that for which the commutation switching-angle was chosen, the stepper motor may run at a velocity very different from the desired velocity. Moreover, when a desired velocity is selected, the corresponding switching angle typically cannot be calculated with precision.
The foregoing prior art stepper motor systems will not accurately control the velocity and position of a load which varies with time. For example, in an ink jet printer, a stepper motor is typically used to drive and position a carriage upon which an ink cartridge is mounted. As the printer prints, the volume of ink in the cartridge decreases thus decreasing the cartridge mass. For the reasons described above, stepper motors configured in open-loop systems or in closed-loop systems utilizing fixed switching angles are not well suited to drive such a carriage.
Another problem associated with closed-loop stepper motor systems is that the shaft encoder and motor rotor must be characterized when the system is initially assembled. In other words, the relative angular positions of the rotor and encoder must be known.